The present invention relates to a method and a device for starting an electric motor.
Brushless direct current motors (also referred to as BLDC motors) are used, for example, as compressor drives in refrigerators. Such electric motors have phase windings, and a rotor which incorporates a permanent magnet. When the phase windings have current flowing through them, they generate a magnetic field which exercises a torque on the permanent magnets and thereby sets the rotor in motion. By actuating the various phase windings in turn, a rotating magnetic field is produced in the motor, which thereby drives the rotor.
Many drives, such as for example compressor drives in refrigerators, have a non-uniform load profile, which means that the load which they drive varies over a complete rotation of the motor, whereby the maximum torque which the motor must apply can be a multiple of the mean torque. When starting up the motor it is therefore advantageous initially to bring the rotor to a clearly defined position, so that the rotor will have already turned through the largest possible angle and have been accelerated to the greatest possible torque before the maximum torque load arises.
One possibility for positioning the rotor is to apply a defined constant current to the phase windings of the motor by actuating it with a particular switch setting, that is by setting it to a particular electrical position. The defined current results in a particular magnetic field, to which the rotor orientates itself with its permanent magnet. However, there is a problem with this approach in that, for motors with more than one pair of poles, the precise mechanical position is not known. Thus, in the case of a 6-pole motor, such as is often used for refrigerator compressors, a particular electrical position corresponds to three different mechanical positions, each of which is offset relative to the others by a rotational angle of 120°. Depending on the starting position, the rotor then adopts the mechanical position which lies closest to this starting position. However, this leads to the motor only being started from the optimal starting position at one of the three possible mechanical positions, whereas at the two other mechanical positions the angle of rotation up to the maximum load is smaller. In this latter case it can happen that when the motor reaches the maximum load it cannot yet apply the necessary torque to overcome the load, and thus stalls and the start-up fails. It may then be necessary to make several start-up attempts in order to start the motor.
Although it would be possible to provide additional sensors, supplying data about the precise position of the rotor, such a solution is associated with additional costs for such sensors.
The publication DE 698 03 885 T2 discloses a control device for electrical motors whereby a control unit issues to the switching facilities a switching rate and switching duration which are defined in such a way that the value of the voltage which is actually applied across the windings is that which corresponds to the rotational speed and the torque which is required of the electric motor, irrespective of the switching state of the switching facilities.
The publication DE 40 09 258 C2 discloses a method and an electronic regulating circuit for starting up a brushless DC motor.
The publication DE 600 25 909 T2 discloses a starting system for an electric motor, which can be attached in the inside of the housing of a hermetically sealed cooling compressor.
Publication DE 102 15428 A1 discloses a method for determining the position of a rotor in a synchronous motor. In this case, “by means of a brake, the rotor of the synchronous motor” is first blocked, or it is held by high static friction. Then, with the brake applied, a plurality of current vectors with different displacement angles is applied to the synchronous motor. The displacement of the rotor can be attributed to the elasticity of the shaft with which the rotor and the brake engage, or the retention force of the brake can be regarded as a spring force for very small displacements.
The publication U.S. Pat. No. 4,565,957 discloses a method and a system for starting a commutated SCR inverter. In this, the rotor is turned to a first position by the production of magnetic fields; it then turned to a second position, where the rotor is completely halted in each of said positions. According to U.S. Pat. No. 4,565,957, the motor can drive a compressor with a large capacity, for an industrial air-conditioning system.